generator: Add support for fixed length arrays

Fixed length arrays are available in gobject introspection and are already converted by the bindinator tool. The array_len attribute was only used in structs. This PR adds support for them as method parameters. 
```xml
      <method name="GetViewport" cname="cogl_get_viewport" shared="true" deprecated="true">
        <return-type type="void"/>
        <parameters>
          <parameter name="v" type="float*" owned="true" pass_as="out" array="true" array_len="4"/>
        </parameters>
      </method>
```
It was missing the array initialization and the out modifier.
```csharp
		public static float[] Viewport { 
			get {
				float[] v;
				cogl_get_viewport(v);
				return v;
			}
		}
```
Will now be generated correctly as
```csharp
		public static float[] Viewport { 
			get {
				float[] v;
				v = new float[4];
				cogl_get_viewport(out v);
				return v;
			}
		}
```